1. Field of the Invention
This invention relates to a system for and method of treating a malfunctioning heart and, more particularly, to such a system and method which effects cardioversion/defibrillation in response to sensing a heart malfunction. The term "hemodynamic parameter", as used herein, means any parameter which may be sensed or determined and either directly or indirectly affects the motion or constituents of blood or performance of the heart within the circulatory system. The invention provides for the cardioverting/defibrillation of a malfunctioning heart as well as the possibility of overcoming a tachycardia manifestation without resorting to either cardioverting or defibrillating the heart.
2. Description of the Prior Art
In recent years, substantial progress has been made in pacemakers and in the development of cardioverting/defibrillating techniques for effectively treating various heart disorders and arrhythmias. Past efforts have resulted in the development of implantable electronic pacemakers and standby cardioverters-defibrillators which, in response to the detection of an abnormal cardiac rhythm, discharge sufficient energy via electrodes connected to the heart to depolarize and restore it to normal cardiac rhythm. An early example of this cardioverting/defibrillating technique is disclosed in U.S. Pat. No. 3,942,536 of Mirowski et al., the technique involving responses to a sensed peak right ventricular systolic pressure dropping to a fixed predetermined threshold level. This known technique did not involve mean pressure changes in either direction from a baseline. Nor did it involve sensing of pressure within any vessels which extends between the heart and lung(s).
Efforts have also been directed toward developing techniques for reliably monitoring heart activity in order to determine whether cardioversion/defibrillation are desirable or necessary. Such techniques include monitoring ventricular rate or determining the presence of fibrillation on the basis of a probability density function (PDF). A system using the PDF technique statistically compares the location of points of a cardiac waveform with the expected locations of points of the normal waveform. When the waveform becomes irregular, as measured by its probability density function, an abnormal cardiac function is suggested. The latter technique is described in U.S. Pat. Nos. 4,184,493 and 4,202,340 both of Langer et al.
A more recent system, as disclosed in U.S. Pat. No. 4,475,551 of Langer et al. utilizes both the PDF technique to determine the presence of an abnormal cardiac rhythm and a heart rate sensing circuit for distinguishing between ventricular fibrillation and high rate tachycardia (the latter being indicated by a heart rate above a predetermined minimum threshold), on the one hand, and normal sinus rhythm or a low rate tachycardia (indicated by a heart rate falling below a pre-determined minimum threshold), on the other hand.
Still further, research in this area has resulted in the development of a heart rate detector system which accurately measures heart rate from a variety of different electrocardiogram (ECG) signal shapes. One such system is disclosed in U.S. Pat. No. 4,393,877 of Imran et al.
Despite these past efforts and the level of achievement prevalent among prior art systems, there are potential difficulties and drawbacks which may be experienced with such devices.
Currently antitachycardia systems detect arrhythmias primarily by sensing rate and perform inadequately in the differentiation of hemodynamically stable from unstable rhythms. These devices, for example, may fire during a stable supraventricular tachycardia (SVT) inflicting pain and wasting energy; damage to the heart may result.
A commonly used implantable antitachycardia device is the automatic implantable cardioverter-defibrillators (AICD) which is commercially available under the model designations 1500, 1510 and 1520 from Cardiac Pacemakers, Inc. whose address is: 4100 North Hamlin Avenue, St. Paul, Minn. 55164. These devices continuously monitor myocardial electrical activity, detecting ventricular tachycardia (VT) and ventricular fibrillation (VF), and delivering a shock to the myocardium to terminate the arrhythmia. The AICD has been shown to reduce the mortality rate in patients with malignant arrhythmias with initial studies at Johns Hopkins Hospital and Stanford Medical Center demonstrating a 50 percent decrease in the anticipated total incidence of death, as reported by Mirowski et al., "Recent Clinical Experience with the Automatic Implantable Cardioverter-Defibrillator", Medical Instrumentation, Vol. 20, pages 285-291 (1986). Arrhythmias are detected by (1) a rate (R wave) sensor and (2) a probability density function (PDF) which defines the fraction of time spent by the differentiated electrocardiogram between two amplitude limits located near zero potential. Presently, the functional window of the PDF is wide to permit the detection of both VT and VF, and therefore, this device functions essentially as a rate-only sensing system. As reported by Mirowski, "The Automatic Implantable Cardioverter-Defibrillator: An Overview", JACC, Vol. 6, No. 2, pages 461-466, (August, 1985), when an arrhythmia fulfills either the rate or PDF criteria, the device delivers Schuder's truncated exponential pulse of 25 Joules some 17 seconds after the onset of the arrhythmia. The device can recycle as many as three times if the previous discharge is ineffective with the strength of the second, third and fourth pulses being increased to 30 Joules. After the fourth discharge, approximately 35 seconds of nonfibrillating rhythm are required to reset the device. The Mirowski et al., supra, and the Mirowski, supra publications set out, in summary form, background material relating to the defibrillating/cardioverting arts against which the present invention was made.
In addition to the standard automatic implantable cardioverter-defibrillator characterized by the above-noted, dual detection algorithm, a variant of the device which features a sensing system that relies only on the analysis of heart rate is also available. This "rate-only" version of the known cardioverter-defibrillator preferred by some investigators, is more sensitive than the dual detection version unit and theoretically less likely to miss ventricular tachycardias with narrow QRS complexes. It is believed that the "rate-only" system, on the other hand, may be too sensitive, delivering cardioverting/defibrillating pulses too often or too soon, no hemodynamic parameter having been taken into consideration.
One problem with current systems is that they function primarily as a rate-only sensing systems and may fire for nonmalignant as well as malignant tachycardias. These firings are not benign; potentially endangering myocardium, wasting energy and inflicting pain on the conscious patient, all distinct shortcomings and disadvantages.
The principal object of the present invention is to provide a system for cardioverting/defibrillating which avoids unnecessary firings, thereby reducing the danger to the myocardium, saving energy and avoiding pain.
Another object of the present invention is to provide an implantable system for cardioverting/defibrillating which avoids unnecessary firings, thereby reducing the danger to the myocardium, saving energy and avoiding pain.
A further object of the present invention is to provide a system for cardioverting/defibrillating which is hemodynamically responsive to change in a hemodynamic parameter, such as pressure, at a site in the circulatory system of a patient.
An additional object of the present invention is to provide a system for cardioverting/defibrillating which is hemodynamically responsive to change in a selected parameter from a baseline (either fixed or varying) and to rate criteria.
Yet another object of the present invention is to provide a method of cardioverting/defibrillating which may be advantageously carried out using a cardioverter-defibrillator constructed in accordance with the present invention.
Yet a further object of the present invention is to provide a method of cardioverting/defibrillating which avoids unnecessary firings thereby reducing the danger to the myocardium, saving energy and avoiding pain.
In accordance with preferred embodiments of the present invention, new sensing algorithms are proposed using hemodynamic or both hemodynamic and rate criteria, the latter being taken in series or parallel. The series configuration algorithm could be effected by detecting rate with an intracardiac, extracardiac, or body-surface R-wave sensor. When rate exceeds the programmed cut-off value, at least one hemodynamic parameter, such as mean pulmonary artery pressure (MPAP), mean pulmonary vein pressure (MPVP), mean pulmonary capillary wedge pressure (MPCWP), mean right atrial pressure (MRAP), mean right ventricular pressure (MRVP), mean central venous pressure (MCVP) mean arterial pressure (MAP) right ventricular systolic pressure (RVSP), right ventricular end diastolic pressure (RVEDP) or right ventricular pulse pressure ((RVPP) departures from a baseline would be monitored. Mean left atrial pressure (MLAP) or mean left ventricular pressure (MLVP) may also be suitable as one or another of the hemodynamic baseline parameters from which changes may be monitored. If mean right arterial pressure (MRAP) or mean right ventricular pressure (MRVP) or mean central venous pressure (MCVP) increases from respective baseline MRAP or MRVP or MCVP baselines within a time period of predetermined duration, indicating hemodynamic compromise, the system would fire. If mean left atrial pressure (MLAP) or mean left ventricular pressure (MLVP) increases respectively from respective baseline MLAP or baseline MLVP within a time period of predetermined duration indicating hemodynamic compromise, the system would fire. If mean arterial pressure (MAP) decreases from baseline MAP beyond a predetermined magnitude indicating hemodynamic compromise the system would fire. If the respective pressure changes were less than the respective predetermined magnitudes, pressures would be monitored to determine if respective changes from the respective mean levels take place, as long as the rate criteria is satisfied. The system and method of the invention as disclosed herein may involve mean pulmonary artery pressure (MPAP), mean pulmonary vein pressure (MPVP) or mean pulmonary capillary wedge pressure (MPCWP). A parallel configuration algorithm in which rate and hemodynamic criteria function simultaneously is also proposed; however, continuous pressure change determination would probably be less energy efficient. Either configuration of algorithm could be adapted to a single catheter consisting of a pressure transducer in either the right atrium or right ventricle and an R-wave sensing electrode or pair of electrodes at the catheter tip in the right ventricle. The hemodynamic information derived from an arterial line, Swan-Ganz catheter (already present in the intensive/cardiac care unit patients), or even an automated mechanical blood pressure cuff could be integrated together with the electrocardiogram to provide a temporary automatic antitachycardia system. Cardioversion-defibrillation could be administered using externally applied patches. Even a noninvasive hemodynamically responsive antitachycardia system is potentially feasible using doppler technology for pressure measurements. The PDF (narrow window of function) and the rate/pressure sensing algorithm could be used simultaneously such that if the rate/pressure criteria are satisfied (indicating hemodynamically significant SVT or VT) the device cardioverters and if the PDF criteria is satisfied indicating (VF) defibrillation results. This pulse delivery system could also be incorporated into a single catheter.
It is to be appreciated that when the pressure criteria is not met, but the rate criteria indicates tachycardia is present, an antitachycardia pacemaker could be enabled in an effort to correct the malfunction.
MAP is an excellent parameter but accurate continuous measurement requires an indwelling arterial catheter or transducer which over time is prone to infection and thrombus formation (with the potential for systemic embolic events). Only recently has an extra arterial external wall pressure sensor for chronic arterial pulse pressure sensings been recorded. MRAP and MRVP appear to relate useful information regarding the hemodynamic state of the particular arrhythmia. MRAP is also an excellent parameter and is easily determined on a continuous basis, using an indwelling cather. If tricuspid stenosis were present, MRVP would probably be more reliable than MRAP. Preliminary observations in the canine model suggest that changes as small as 3 mmHg for MRAP and MRVP and as small as 15 mmHg for MAP are significant and can be used in carrying out the present invention.
The rate/pressure sensing algorithms could also help integrate a cardioverter-defibrillator with an antitachycardia pacemaker. The hemodynamic function would determine which of these devices to engage. For example, when a hemodynamically significant tachycardia is detected the cardioverter-defibrillator would be used to terminate the arrhythmia. When a hemodynamically stable tachycardia is sensed the antitachycardia pacemaker would attempt to terminate the arrhythmia using such methods as overdrive, burst, or extra stimulus pacing, incremental or decremental scanning, or ultra-high frequency stimulation. If the tachycardia was accelerated, this would be detected by the rate/pressure sensing algorithm and cardioverted or defibrillated. With a pacemaker present, a bradycardia failsafe could be built into the system.
The adaptation of a hemodynamic parameter to the sensing system of antitachycardia devices appears to be a logical improvement to its present function. MRAP, MRVP, RVSP, RVPP and RVEDP are easily determined parameters (via the transvenous route) and appear to relate important hemodynamic information. MAP is an easily measured parameter in the intensive/cardiac care unit setting and could be integrated together with the electrocardiogram to form a temporary automatic antitachycardia system. A long term MAP sensor may be feasible with recent advances. A rate/pressure sensing algorithm, designed either in series or parallel, could be integrated with the PDF system such that hemodynamically significant SVT, VT, and VF would be detected. The rate/pressure sensing algorithm could also be applied to a combined cardioverter-defibrillator and antitachycardia pacemaker.
In its apparatus aspect, the invention can be seen as being in a system for treating a malfunctioning heart of the type which includes storage means for storing electrical energy and electrode means for electrically coupling the storage means to the heart. Determining means are provided for determining at least one hemodynamic parameter. Means provide a first signal representative of baseline level for the parameter. Means responsive to output from the determining means develop a second signal representing current level of the parameter over a period of given duration. Means responsive to output from the means for providing the first signal and output from the means for developing the second signal charge and enable discharge of the electrical energy stored by the storage means across the electrode means upon change in the current level of the parameter of at least a predetermined amount from the representative baseline level for the parameter.
The means providing a signal representative of baseline level for the parameter may be constituted by means providing a signal representative of a fixed baseline level for the parameter.
The means for providing a first signal representative of baseline level for the parameter may be constituted by means for developing a variable first signal representative of baseline for the parameter over a period of predetermined duration which is greater than the period of given duration.
In its method aspect, the invention can be seen as a method of treating a malfunctioning heart which includes providing a representation of baseline for a hemodynamic parameter and determining current level of the parameter over a period of given duration. Therefore, the method involves delivering cardioverting/defibrillating electrical energy to the heart in response to change of at least a predetermined magnitude in the current parameter from the baseline for the parameter.
The step of providing a representation of baseline for the parameter may be constituted by providing a fixed representation of baseline for the parameter.
The step of providing a representative of baseline for the parameter may be constituted by providing a varying representation of baseline for the parameter over a period of predetermined duration which is greater than the period of given duration.
The novel features that are considered characteristic of the invention in its method and system aspects are set forth with particularity in the appended claims. The invention itself, however, both as to its organization and its method of operation, together with other objects and advantages thereof is to be understood from the following description of illustrative embodiments, when read in conjunction with the accompanying drawings, wherein like reference numerals refer to like components.